In Matthew 12:33, Jesus said,
"Either make the tree good and the fruit good, or make the tree bad and its fruit bad (harmful, rotten); for by the fruit, the tree is known."
Jesus' point is this: What's inside comes out. The context has a lot to say about how that all works, but for today I just want to focus on the one statement: "By the fruit the tree is known."
It's more important to observe actions than motives, and it's most important to consider all of someone's actions (or, as here, teachings), not just one. A tree can produce a few pieces of bad fruit (when I was a youngster, we had a fig tree that did just that), but mostly produce good fruit. When you look at a tree, Jesus said, don't look at the leaves or the trunk or branches. Instead, look at the fruit. It's the same as saying,
"don't look at a person's wealth, his words, or his status." He may be a big tree, lots of leaves, nice branches--but what is his fruit? Now fruit is something you EAT. So if you go, say, to an oleander bush, and take off some of its fruit (the fruit is tiny), and eat it, what will happen? Well, oleander is a variety of hemlock, and you will die.
So perhaps the better question is, "can you eat the fruit of this tree?" What is the produce like? Is this a peach tree, or a loco weed?"
What does eating the fruit of this tree do to you? Does it make you healthy and strong? or is it poison? If it weakens you, poisons you, makes you worse, then the fruit is not good.
This is really the question Jesus is asking--and he's asking it about the teaching and life of the Jewish leaders of his day (I'm basing my view on the context). "What happens when you eat their fruit?" Meaning, "what happens when you imbibe and follow their teaching?" What do you become? Does their fruit poison your life? Or does it fill your life with health and strength, and make you a better, stronger person?
This is really the question all of us must deal with in our lives. To whom do we listen? Are the people who teach us "producing edible teaching fruit?" Or do they produce fruit that poisons our lives?
"By their fruit you shall know them."
and here, "a tree is known by its fruit." Interestingly, the word "known" is in the present tense in the original, leading me to think that this is an ongoing process of knowing, so that we evaluate the tree's fruit over time, as we decide whether or not it's healthy for us to use.
This makes the decision of who to allow inside my head very personal. Some people have what amounts to toxic teaching--it poisons the mind. Others have teaching which beautifies and strengthens it. The Jewish leaders of Jesus' day were experts at disguising the poisonous things they taught and attitudes they held with many fine words, but Jesus said, "Look at them! What's the effect of their teaching, if you allow them inside your head?" (hint: for an evaluation of their "fruit," see Matthew 23).
We have to do the same. It's our job to protect ourselves, and "eat the good fruit."
Our motto needs to be, "I want to find the peach, not the poison."